Method of performing a surge-boil step during the heating period of a lignocellulosic material cooking stage

ABSTRACT

In the periodic cooking of lignocellulosic material in a digester, the homogeneity of the digester contents can be improved by the interposition of at least one surge-boil during the cooking process which is effected by reducing the pressure in the digester together with the direct introduction of steam into the bottom of the digester, simultaneous with pressure reduction, at a rate of at least 0.1 ton steam/minute.

United States Patent Backlund 1*Sept. 23, 1975 1 METHOD OF PERFORMING ASURGE-BOIL [58] Field of Search 162/42. 6]

STEP DURING THE HEATING PERIOD OF A LIGNOCELLULOSIC MATERIAL ReferencfiCited COOKING STAGE UNITED STATES PATENTS 7 Inventor; Su ()l f iBacklund, 1.831.206 11/1931 Swanson ct al. 162/61 X Sundsvan, Sweden1905.050 4/1933 Pelot et al v 162/6] 3.298.889 1/1967 Laakso 162/42 xNotice: The portion of the term of this 3 467 573 9/1969 Vogel 162/42 Xpatent subsequent to Feb. 19, 1991, has been disclaimed. PrimaryE.\aminerS. Leon Bashore [22] Filed: June 8, 1973 AssistantExaminer-Arthur L. Corbin pp 368,032 57] ABSTRACT R l t d [1,3, A li tiD t In the periodic cooking of lignocellulosic material in a [63]Continumiommpan of No. 209,883 20 digester, the homogeneity of thedigester contents can 1971, PM No. 3391137. be improved by theinterposition of at least one surgeboil during the cooking process whichis effected by 30 Foreign Application p i it Data reducing the pressurein the digester together with the 14 1973 S 7306760 direct introductionof steam into the bottom of the diay v we en v gestcr, slmultaneous w1thpressure reduction, at a rate 52 us. c1 162/42; 162/61 of at eastSteam/mum- [5 l] Int. Cl. DZIC 7/12 I Claim, 2 Drawing Figures US PatentSept. 23,1975 Sheet 2 0f 2 3,907,631

FIG.2

METHOD OF PERFORMING A SURGE-BOIL STEP DURING THE HEATING PERIOD OF'ALIGNOCELLULOSIC MATERIALCOOKING STAGE 'This application is aeontinuation-in-part' ofmy copending Application Ser. No. 209,883 filedDec. 20, 1971 now U.S.Pat. No. 3,793,137;

This invention relates to a'method' of'obtaining homogeneous digestionin the periodic cooking of a lignocellulosic material.

In all pulp manufacturingprocesses uniform cooking is of greatimportance, with respect both to economy and to the quality of thepulp.Thc' reasons 'for nonuniform cooking vary with the process type. TheSwedish Pat. Spec. No. 545,885 enumerates the most important factorsaffecting the homogeneity at periodic cooking and it describcs'a methodwhich hasas its object to eliminate the most dominating one of thesefactors, viz, the cooking liquor circulation. I I

Experiences from a mill installation prove that the techniques appliedaccording to the method, i.e., surgeboil, has a very positive effect onthe homogeneity of the cooking.

It was found, conjunction with the surge-boil method was the gassingcondition of the digesters, i.e.. the overpressure of air and othergases in the digester when the surge-boil commences during the cookingprocess. An effective degassing of the digesters prior to thesurge-boils is one condition for obtaining a full surge-boil effect,i.e., a fully satisfactory digestion uniformity in the digester.

1t has proved difficult, however, to carry out a complete degassing ofthe digesters in time before the surge-boil stage, particularly in thecase of rapid heating processes.

When experiments were made for overcoming these difficulties, it wasfound by surprise that the system could be rendered less sensitive tooverpressure at the.

surge-boils by supplying direct steam to the digester bottom.

The surge-boils, thus, are brought about according to the presentinvention by pressure reduction in the digester and simultaneous directsupply of steam to the digester bottom. Thereby the surgeboils becomemore efficient, especially during rapid heating processes. The steamsupply is to be substantial, i.e., at least 0.1 ton steam/minute,preferably 0.1 0.5 ton steam/minute.

The pressure reduction in the digester bringing about a surge-boil iseffected either by degassing from the digester top or by supply ofcooling liquor, for example cooking liquor or water, to the digestertop.

At least one of said surge-boil steps is carried out during the heatingperiod of the cooking stage at a point of time when the chlorine numberof the pulp is 30-20.

The duration of the surge-boil periods may vary, but should be at least5 seconds. A normal period is seconds minutes, suitably 1 10 minutes andpreferably 2 5 minutes.

FIG. 1 shows a periodic sulphate digester with a circulation system forindirect heating, and

FIG. 2 shows such a digester with a system for direct heating withsteam.

The invention is described in the following by way of two examples withreference to the drawings. Although the examples refer to sulphatecooking, the method achowever, that an important factor in equipped witha circulation system for indirect heating 1 according to'FlG. 1. Thedigester is charged by means of a liquor packing method with about23tons of wood chips calculated on the dry basis. 'White liquor and blackliquor are charged in an amount such that the alkali charge is200'kgactive alkali (NaOH) per ton of absolutely 'dry wood, andthe'woodfili'quor ratio is 1:35

The circulation pump lis started already whilst the chips are beingcharged, and it sucks liquor from the digester through a strainer unit2. The cooking liquor is circulated through the heat-exchanger-3 forheating arid is distributed after the heat-exchanger so that 8071 of thevolume flow is passed to the top of the digester via a conduit 4 and theremaining 20% is passed to the digester bottom 'via a conduit 5 Theliquor circulated to the top of the digester is distributed over thechips by means of centrifugal pressure sprays 6. The heating to 170 C iseffected within 1 50 minutes by indirect heating with steam in the heatexch anger 3 and during the surge-boils also by direct heating withsteam. The temperature in the top circulation conduit 4 duringtheheating of the cook having reached 153 C., thc first surge-boil iscarried out. This is effected in such a manner that the valve 7 isclosed automatically by a time clock which was started at the start ofthe cooking process.

Thirty seconds after the closing of valve 7. the dcgassing valve 8 isopened whereby the digester is degassed through the conduit 9, and atthe same time the steam valve 10 is opened for supplying direct steamthrough a conduit 11. After 5 minutes the valves 8 and 10 are closed,and the valve 7 is opened.

The heating of the cook can thereafter proceed in a normal way. Duringsaid degassing and simultaneous steam supply to the digester bottom. asurge-boil takes place in the digester, which owing to the steam supplybecomes relatively insensitive to over-pressure in the digester. Thesame surge-boil step is repeated also at C. and at 167 C. The cook isheld at the maximum temperature of 170 C. for about 1 hour. Subsequentto the in-takc of about 10 m cooling liquor. the digester is blown.Blowing time 30 minutes.

The chlorine number of the pulp was 5.7 and the shive content whenscreening an average sample from the cook in a Wennberg-strainer havinga slit width of 0.20 mm was 1.0%. The shive content of a cook, at whichthe surge-boil technique was not utilized, was 5.2% at a chlorine numberof 5.7.

EXAMPLE 2 The cooking is carried out in a periodic sulphate digesterhaving a volume of 125 m according to FIG. 2. The digester is equippedfor direct heating by steam. The digester is charged with wood chips,which are packed by means of a steam packing unit so that the amount ofchips in the digester is about 25 tons of chips calculated on the drybasis. White liquor and black li quor are charged in a amount such thatthe alkali charge is 200 kg of active alkali (NaOH) per ton ofabsolutely dry wood, and the wood:liquor ratio is 1:30

The cook is heated by the supply of direct steam through the conduit 14to the bottom of the digester. The steam supply iscontrolled by thevalve 15 so that the heating to 170C is effected within 30 minutes.During the cooking step de-gassing of air, turpentine, etc.,' takesplace all the time in a normal manner through the conduit 16 and withde-gassing control by the valve 17.

The temperature in the top of the digester having reached 155C, thefirst surge-boil is carried out. This is effected by opening the valve13 for minutes so that a substantial de-gassing takes place through theconduit 12. The surge-boil .de-gassing is to be so substantial that ateach surge-boil 0.5 5% of the total liquor amount is removed from thedigester. The de-gassed liquor thereby removed comprises about 5 25%steam, the remainder being foam and liquor droplets.

The heating of the cook by direct steam continues during thesurge-boil'pcriod in a normal manner.

Thev temperature in the digester having reached 170C, the steam supplyis interrupted by closing the valve 15.

After 5 minutes at 170C the second surge-boil is effected by againopening the valve 13 for 5 minutes and simultaneously also chargingsteam by opening the valve 15. The steam is to be charged in an amountof the magnitude 0.1 0.5 ton steam/minute.

After completing the second surge-boil the valves (13) and are againclosed.

The chlorine number of the pulp was 6.0 and the shive content whenstraining an average sample from the cook in a Wennberg-strainer withslit width 0.20 mm was 1.5%. For a cook in which the surge-boiltechniquewas not utilized, the shive content was 7.0% at a chlorine number, of6.0.

'1 claim:

1. ln a method of improving the homogeneity in periodic cooking oflignocellulosic material in a digester including a heating stage andhaving a heating system constructed in a manner to permit steam to besupplied directly to the bottom of the digester wherein at leastonesurge-boil step having a duration of from 10 min utes to 5 seconds iscarried out, during the heating period of the cooking stage at a pointof time when the chlorine number of the pulp is 3020, by pressurereduction in the digester, said pressure reduction being obtained bycarrying out at least one of the steps of (a) removing gas from'the topof the digester and (b) supplying cooling liquid to the top of thedigester, the improvement which consists in enhancing the surge-boilstep by simultaneously with the pressure reduction directly supplyingsteam to the digester bottom in an amount of at least 0.1 tonsteam/minute.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PATENT NO.3,907,631

DATED 3 September 23,1975 |NVENTOR(5) 1 Sture Olof Erik Backlund It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

[73] Assignee: SCA Projekt Aktiebolag Signed and Scaled this ninth a O Dy f December 1975 Attest:

RUTH- C. MApSON C. MARSHALL DANN Aniflmg ff Commissioner oflarents andTrademarks

1. IN A METHOD OF IMPROVING THE HOMOGENEITY IN PERIODIC COOKING OF LIGNOCELLULOSIC MATERIAL IN A DIGESTER INCLUDING A HEATING STAGE AND HAVING A HEATING SYSTEM CONSTRUCRED IN A MANNER TO PERMIT STEAM TO BE SUPPLIED DIRECTLY TO THE BOTTOM OF THE DIGESTER WHEREIN AT LEAST ONE SURGE-BOI STEP HAVING A DURATION OF FROM 10 MINUTES TO 5 SECONDS IS CARRIED OUT, DURING THE HEATING PERIOD OF THE COOKING STAGE AT POINT OF TIME WHEN THE CHLORINE NUMBER OF THE PULP IS 30-20, BY PRESSURE REDUCTION IN THE DIGESTER, SAID PRESSURE REDUCTION BEING OBTAINED BY CARRYING OUT AT LEAST ONE OF THE STEPS OF (A) REMOVING GAS FROM THE TOP OF THE DIGESTER AND (B) SUPPLYING COOLING LIQUID TO THE TOP OF THE DIGESTER, THE IMPROVEMENT WHICH CONSISTS IN ENHANCING THE SURGE-BOIL STEP BY SIMULTANEOUSLY WITH THE PRESSURE REDUCTION DIRECTLY SUPPLYING STEAM TO THE DIGESTER BOTTOM IN AN AMOUNT OF AT LEAST 0.1 TON STEAM/MINUTE. 